The present invention provides a method for discriminating an organic farmland from a conventional farmland, comprising; (a) measuring an amount of nitrogen isotope in a soil sample of a farmland; (b) calculating a nitrogen isotope index according to the formula (I) using the resultant nitrogen isotope value and (c) determining whether the farmland belongs to an organic farmland or a conventional farmland. According to the present invention, it is possible to objectively discriminate an organic farmland from a conventional farmland or discriminate an organic crop from a conventional crop.